Total sleep deprivation, partial sleep deprivation, and shifting the time of the sleep period several hours earlier than usual can improve depression and can induce mania. Also, recovery sleep after these interventions can induce depression. Thus, changes in the timing and duration of sleep could trigger episodes of depression and mania during the natural course of affective illness, and manipulations of sleep could be used to treat and to prevent affective episodes. This project was designed to investigate the importance of timing of sleep in patients' clinical responses to sleep manipulations. In particular, we hypothesized, on the basis of previous findings, that being awake or asleep in the second half of the night was critical for the responses. Identification of a critical circadian phase for these effects of sleep would provide an important clue to their mechanisms and would facilitate the design of effective and practical sleep deprivation treatments for depression. The antidepressant response to partial sleep deprivation early in the night (PSD-E) was compared with the response to partial sleep deprivation late in the night (PSD-L) in 16 drug-free depressed inpatients using a balanced order crossover design. PSD-L had a significantly greater antidepressant effect than PSD-E. The response to PSD-L was sustained and enhanced by a second night of treatment. Patients had significantly shorter sleep durations and reduced REM sleep on PSD-L that did not occur on the PSD-E condition. There was a significant negative correlation between response to PSD and sleep duration, and in particular, REM sleep duration, on the late sleep deprivation condition. Thus, the amount and timing of sleep appear to be factors in the response to PSD, and the effect of this procedure on REM sleep may be important.